The invention relates to optomechanical radiant energy detectors and specifically to a micromechanical structure which responds to radiant energy by interrupting the transmission of waveguided light, thereby modulating the waveguided light to provide an output signal.
The worldwide infrared (IR) camera market is projected to grow at over 25% per year, from $2 billion in 1999 to $8 billion in 2008. The key growth drivers are low cost, high-performance IR detectors. These detector modules will expand existing IR markets and create new ones, both in military markets and to a greater extent in the commercial sector.
One example of a key market application is predictive maintenance, currently, this market has a valuation of over $185 million. IR (Infra Red) sensors in this application are used to generate heat patterns that have been found to correlate with pending component failure. In the security and surveillance market, elimination of darkness defeats a perpetrator""s natural advantage. In the $105 million process monitoring market, heat patterns in, for example, paper production correlate with paper quality and waste reduction.
The development of a high resolution, high sensitivity imager has the potential to greatly expand the marketplace. This can be attributed directly to reduced systems costs derived from savings in optics and electronics that result from leveraging the high performance of the new devices. The cost of IR optics for camera systems can be reduced in an inverse proportion to an increase in device performance and in proportion to a reduction in pixel size. As the costs are reduced, new unrealized markets may be able to benefit from the use of IR imaging and these high performance devices. These applications include the home security, marine, and automobile industries. Military users will also benefit from the growth in the commercial marketplace resulting from these high performance detectors in the form of lower costs, better reliability, and improved availability of sensors that meet military specifications.
Infrared imaging devices based upon photon-to-electron conversions can be extremely sensitive; however, a parallel process of thermal generation of electrons can produce significant detector noise. Consequently, photoelectric devices need to be cooled for effective use for infrared imaging. This leads to the use of dewars and cooling devices that add significant weight, bulk, and energy consumption to the imager. Thermal detectors, in contrast to photoelectric detectors, do not need to be cooled since they convert the broadband heat absorbed directly into some measurable signal. A significant gap exists, however, between the performance of the best of the current generation of uncooled IR imagers and that of cooled sensors. These xe2x80x9cuncooledxe2x80x9d detectors are limited by the ability to thermally isolate them from their surroundings, by the amount of noise they introduce in their detection process, and by the readout noise introduced by electronics. A 20 mK NExcex94T (the temperature difference that gives a signal equal to the noise) appears to be the best uncooled sensitivity achievable for a microbolometer. In addition, manufacturing problems at this performance level are severe. With sufficient thermal isolation and reduced sensing noise, the sensitivity of thermal detectors can approach or even exceed that of photoelectric converters without the need for the mass, volume, and power required for cooling.
Currently, several types of thermal detectors are used in uncooled imaging systems. The characteristics of some of these systems are summarized below.
Resistive Microbolometers: Currently available Vanadium Oxide devices have an NExcex94T of about 50 mK, with the best achievable NExcex94T of about 20 mK. Magneto resistive devices can have conversion sensitivity of up to about 15%, although none has yet been built into a practical device. Vanadium Oxide has a conversion sensitivity of about 2.5% per degree and has a noise limitation, in that Johnson noise is created when the resistor is read out with a current. These limitations lead to theoretical limitations in the best NExcex94T that these detectors can achieve. In addition, there are cost implications associated with exotic material fabrication.
Ferroelectric Microbolometers: The best BST (BaSrTiO3) detectors on the market have good conversion sensitivity but poor thermal isolation, which results in an NExcex94T close to that of resistive detectors. This is because the bulk material has high thermal capacity. In order to scan the detector at a 30-Hz frame rate, therefore, the thermal time constant must be reduced to about 10 msec by lowering the thermal resistance. There is continuous research in thin film ferroelectric (TFFE) materials such as lead titanate that promise to deliver high conversion sensitivity with lower thermal capacity.
Microcantilever with Capacitor Readout: Measured conversion sensitivity of this device is 50% per degree, achieved through mechanical advantage. Johnson noise is eliminated so that the sensitivity is limited by the noise in the read-out amplifier. Thermal isolation has been measured in thin film amorphous hydrogenated silicon carbide at 10 times greater than silicon nitride. The theoretical NExcex94T limit is less than 5 mK.
Microcantilever with Field Emitter Readout: The conversion sensitivity is greater than 100% per degree and may be as high as 1,000% per degree. However, the field emission process is very noisy so that the predicted NExcex94T using this method is about 80 mK. Future research into the theoretical noise limitations of field emitters would be required to optimize this structure. Thermal isolation is. equivalent to the other methods of isolating microcantilevers.
Microcantilever with Reflected Optical Readout: The conversion sensitivity can vary dramatically depending on the particular geometry. At present it is not clear what noise source is dominant for these optical readouts. The thermal isolation is the same as for other microcantilever methods. The conversion sensitivity can vary dramatically depending on the particular geometry.
The present invention is an optomechanical radiant energy detector comprising a substrate having a principal waveguide and a cantilever arm including a radiant energy absorber, an anchor, an isolation arm, an actuator, and a coupling waveguide. The anchor is coupled to the substrate, and the isolation arm is coupled to the anchor and the radiant energy absorber. The coupling waveguide is coupled to the actuator proximate to the principal waveguide. Also, the actuator is coupled to the isolation arm. Each waveguide emits an evanescent field. The evanescent fields are coupled in response to motion of the cantilever arm induced by incident energy upon the radiant energy absorber.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.